Method of laser annealing semiconductor layer and semiconductor devices produced thereby

ABSTRACT

A laser annealing method includes forming a nitrogen-doped layer on a semiconductor layer, the nitrogen-doped layer having a nitrogen concentration of at least 3×10 20  atoms/cc, irradiating a first area of the nitrogen-doped layer in a low oxygen environment with a laser beam and irradiating a second area of the nitrogen-doped layer in a low oxygen environment with a laser beam, a part of the second area overlapping with the first area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Singaporean Patent Application No. 200802817-7, filed Apr. 9,2008, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of laser annealing asemiconductor layer and semiconductor devices produced thereby.

2. Description of the Related Art

The production of semiconductor devices commonly includes one or moresteps of laser annealing a semiconductor layer. Typically, but notexclusively, laser annealing is carried out to crystallize anon-single-crystal semiconductor layer, such as an amorphoussemiconductor layer of the semiconductor device before it is furtherprocessed.

For example, in the production of flat panel display devices such asliquid crystal displays (LCDs) and organic light emitting diode (OLED)displays, an amorphous silicon (a-Si) layer may be laser-annealed toform a polycrystalline silicon (p-Si) layer, by using which thin-filmtransistors (TFTs) that control the pixels of the LCD or OLED displaymay be formed.

Depending on the size of the layer to be annealed and the dimensions ofthe irradiating laser beam, laser annealing may be carried out in two ormore sweeps or scans. For example, in the case where laser annealing iscarried out on an a-Si semiconductor layer including a 2×2 array ofproduct regions, the laser beam 104 that is used for annealing has aneffective working area having a predetermined length x and width y.Typically, the maximum length x of the laser beam is insufficient toirradiate the entire surface of the array. In most cases, therefore, thelaser beam is first scanned across a first area of the array, and thenthe laser beam is scanned across a second area of the array in the samedirection. For instance, as disclosed in Jpn. Pat. Appln. KOKAIPublication No. 7-249591, in order to ensure that the entirety of thearray is annealed, the scanning of the laser beam in the second area isoverlapped with the first area, creating an overlap region.

Due to the dual exposure to laser annealing, the overlap regiongenerally exhibits undesirable characteristics, such as unacceptablevariations in electrical or physical characteristics. To ensure thatthese undesirable characteristics do not affect the productsmanufactured from the array, the overlap region is conventionallyarranged outside of the product regions.

As described above, since the conventional overlap region of thesemiconductor layer is not usable for the fabrication of the product,due to the problems in electrical or physical characteristics of theoverlap region, this part of the array becomes useless. Hence it isdifficult to efficiently fabricate products from one array. In addition,if a product of a greater size is to be annealed, an overlap region oflaser annealing occurs in the product, leading to the difficulty offabrication.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of theabove-described points, and its object is to provide a laser annealingmethod which enables annealing of a semiconductor layer without causingundesirable variations in electrical or physical properties of asemiconductor layer, can improve the efficiency of fabrication, andenables manufacture of large-sized products, and to provide asemiconductor device which is produced by this method.

According to an aspect of the invention, there is provided a method oflaser annealing a non-single-crystalline semiconductor layer, thenon-single-crystalline semiconductor layer including a product region,the method comprising:

-   -   forming a nitrogen-doped layer on the non-single-crystalline        semiconductor layer, the nitrogen-doped layer having a nitrogen        concentration of at least 3×10²⁰ atoms/cc;    -   irradiating a first area of the nitrogen-doped layer in a low        oxygen environment with a laser beam; and    -   irradiating a second area of the nitrogen-doped layer in a low        oxygen environment with a laser beam, a part of the second area        overlapping with the first area.

According to another aspect of the invention, there is provided asemiconductor device comprising a laser-annealed semiconductor layer,the laser-annealed semiconductor layer having a nitrogen concentrationof at least 3×10²⁰ atoms/cc at a surface thereof.

According to the above method, the gap between product regions on thearray no longer needs to function as an overlap region, and the distancetherebetween can be decreased. Thereby, useless semiconductor surfacesor semiconductor substances can be reduced.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a flowchart illustrating the process flow of a methodaccording to an embodiment of the present invention;

FIGS. 2A, 2B, and 2C are cross-section diagrams of a semiconductor layerbeing annealed in accordance with an embodiment of the method;

FIGS. 3A, 3B, and 3C are cross-section diagrams of a semiconductor layerbeing annealed in accordance with another embodiment of the method;

FIG. 4A is a diagram of an array of product regions undergoing laserannealing according to a comparative example;

FIG. 4B is a perspective view showing an array of product regionsundergoing laser annealing in accordance with one embodiment of themethod;

FIG. 5A is a cross-section diagram of the overlapping of two laser scansduring the laser annealing by the method of the comparative example;

FIG. 5B is a graph showing the results of the laser annealing shown inFIG. 5A;

FIG. 6A is a cross-section diagram of the overlapping of two laser scansduring laser annealing in accordance with one embodiment of the method;

FIG. 6B is a graph showing the results of the annealing of theembodiment shown in FIG. 6A;

FIG. 7 is a cross-section diagram showing a semiconductor deviceaccording to an embodiment of the invention;

FIGS. 8A, 8B, 8C, and 8D are cross-section diagrams illustrating themethod of manufacturing the semiconductor device; and

FIGS. 9A, 9B, and 9C are cross-section diagrams illustrating the methodof manufacturing the semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

A laser annealing method and a semiconductor device according to anembodiment of the present invention will now be described in detail withreference to the accompanying drawings. FIG. 1 generally illustrates amethod of laser annealing a semiconductor layer having one productregion, according to an embodiment of the invention. The term “productregion” as used in this specification denotes a region of asemiconductor layer on or in which a semiconductor device or componentsof a semiconductor device have been or will be formed. Non-limitingexamples of semiconductor devices include transistors, diodes, andsensors.

At step 200 of FIG. 1, a nitrogen-doped layer having a nitrogenconcentration of at least 3×10²⁰ atoms/cc is formed on the semiconductorlayer. Laser irradiation in a low oxygen environment is then carried outat step 202 for a first area of the nitrogen-doped layer, and at step204 for a second area of the nitrogen-doped layer. A part of the secondarea overlaps with a part of the first area in at least one productregion. A “low oxygen environment” for the purposes of thisspecification is an environment in which the volume of oxygen is lessthan 2% of the total volume of the environment. In one form, the lowoxygen environment has around 0.3% oxygen. In another form, the lowoxygen environment has substantially 0% (i.e., 10 ppm or less) oxygen.

One embodiment of the method of FIG. 1 will now be described withreference to FIGS. 2A, 2B, and 2C, a cross-section of an examplesemiconductor layer 300 having a product region 302 is shown attached toa substrate 304. Skilled persons will appreciate that a plurality ofproduct regions 302 may be provided instead of a single product region302. The substrate 304 may be, for example, a glass, silicon, quartz orsapphire substrate, and the semiconductor layer 300 may be anon-single-crystal semiconductor layer, for example, an amorphoussilicon (a-Si) layer, a microcrystalline silicon (p-Si) layer, andpolycrystalline silicon (p-Si) layer. The semiconductor layer 300 may beformed on the substrate 304 by sputtering, chemical vapor deposition(CVD, including specific types of CVD such as low-pressure CVD, plasmaCVD, etc.) or like processes, as will be known to skilled persons.

It should be noted that the illustrated arrangement of semiconductorlayer 300 in the Figure is not essential and that the cross-section viewshown does not represent the entirety of the semiconductor layer and itsarrangement over a substrate. For example, in central regions, there maybe additional layers provided above or beneath the semiconductor layer300. Non-limiting examples of such additional layers include one or moremetal layers, one or more additional a-Si layers, or one or more siliconoxide (SiO) and/or silicon nitride (SiN) layers.

Referring now to FIG. 2B, a nitrogen-doped layer 306 is formed on thesemiconductor layer 300 in accordance with step 200 of FIG. 1. Thenitrogen-doped layer 306 has a nitrogen concentration of at least 3×10²⁰atoms/cc. The applicant has found this to be a desirable concentrationthat reduces the occurrence of undesirable variations incharacteristics, such as unacceptable ablation (i.e., loss of materialfrom the surface) of the semiconductor layer when the laser annealingprocess is carried out in a low oxygen environment. The reason for laserannealing in a low oxygen environment will be explained later in thisspecification.

In this embodiment, the nitrogen concentration is between 3×10²⁰ and3×10²² atoms/cc. In another embodiment, the nitrogen concentration isbetween 5×10²⁰ and 5×10²¹ atoms/cc.

The nitrogen-doped layer 306 may have a thickness in the range of 1 to30 or 5 to 15 nm, for example. In one example, the nitrogen-doped layer306 has a thickness of around 10 nm. The range of 1 to 30 nm has beenfound by the applicant to allow large and uniform grain size to beobtained after annealing, which improves the electron mobility of theannealed semiconductor layer, while ensuring ablation is reduced oravoided altogether. The specific range of 5 to 15 nm has been found bythe applicant to allow the thickness of the nitrogen-doped layer 306 tobe more easily controlled during mass production of semiconductordevices, thus allowing improved operating margins. With regard to grainsize, where the above ranges of thickness of the nitrogen-doped layer306 are implemented, the applicant has found it possible to obtain adesirably large and uniform grain size (i.e., greater than 0.2 μm) afterannealing. In one example, the grain size after annealing is not lessthan 0.3 μm.

In the exemplary embodiment illustrated in FIG. 2B, the nitrogen-dopedlayer 306 is provided by forming a further a-Si layer on the a-Sisemiconductor layer 300, and doping the further a-Si layer withnitrogen. In other words, the semiconductor layer 300 is a first a-Silayer and the nitrogen-doped layer 306 is a second a-Si layer that isdoped with nitrogen. The second a-Si layer may be deposited bysputtering, chemical vapor deposition (CVD including specific types ofCVD such as low-pressure CVD, plasma CVD, etc.) or like processes, andmay be doped with nitrogen using a film forming process, an ionimplantation process after the film forming, or a plasma doping process.

Where CVD is used, the second a-Si layer may be deposited in a CVDchamber and doped with a nitrogen substantially at the same time as thelayer forming, by introducing silane (SiH₄) gas and a nitrogen-based gas(e.g., N₂O) in the CVD chamber. For instance, the silane gas may beintroduced at a first flow rate and N₂O gas may be introduced at asecond flow rate. In this form, the nitrogen concentration and thethickness of the second a-Si layer may be controlled by controlling oneor both of the first flow rate and the second flow rate. Also, in thisform, the nitrogen-doped layer may be provided in a single step, thusreducing the time and cost associated with forming the nitrogen-dopedlayer. It is, however, not essential to both forming and dope the seconda-Si layer substantially at the same time. For instance, the second a-Silayer may be formed first and then, nitrogen may be doped in the seconda-Si layer by ion implanting or plasma doping nitrogen into the layer.In this form, the nitrogen concentration of the second a-Si layer may becontrolled by controlling the energy used in the ion implantation orplasma doping process.

Once the nitrogen-doped layer 306 is formed, a first area of thenitrogen-doped layer 306 is irradiated with a laser beam in a low oxygenenvironment, in accordance with step 202 of FIG. 1, so as to anneal thefirst area and that portion of the a-Si semiconductor layer 300 which issubstantially beneath the first area. The laser irradiation on the firstarea is represented in FIG. 2B as solid arrows 308.

A second area of the nitrogen-doped layer 306 is then irradiated with alaser beam in a low oxygen environment, in accordance with step 204 ofFIG. 1, so as to anneal the second area and that portion of the a-Sisemiconductor layer 300 which is substantially beneath the second area.The laser irradiation on the second area is represented in FIG. 2B asdashed arrows 310. In one form, each of the annealing steps is carriedout in an environment having about 0.3% oxygen. In another form, each ofthe annealing steps is carried out in an environment havingsubstantially 0% oxygen (oxygen concentration of 10 ppm or less).

A low oxygen environment is desirable during laser annealing as theconcentration of oxygen in the annealing environment is directlyproportional to the grain protrusion (i.e., a defect in the form ofsurface roughening) that is present on the semiconductor layer afterannealing. This grain protrusion leads to deterioration in electricaland physical characteristics. In other words, a lower concentration ofoxygen yields lower grain protrusion, which in turn yields desirable (orimproved) electrical and physical characteristics.

However, laser annealing in a low oxygen environment typically increasesthe occurrence of other defects, such as ablation. By using anitrogen-doped layer having a concentration of at least 3×10²⁰ atoms/ccon the semiconductor layer to be annealed, the applicant has found thatlaser annealing can be carried out in a low oxygen environment whilesuppressing the occurrence of an ablation. This will be described infurther detail later in this specification.

Referring back to FIG. 2B, and as highlighted earlier, the laserannealing (by laser irradiation) of the first and second areas of thenitrogen-doped layer 306 is carried out such that a part of the firstarea and a part of the second area overlap in the product region 302.The benefits of having the overlap arranged in this manner will beapparent later in this specification.

Once the laser annealing steps have been carried out, the resultingarrangement is as shown in FIG. 2C, where the a-Si in the semiconductorlayer 300 and the a-Si the nitrogen-doped layer 306 together form apolycrystalline silicon (p-Si) layer 312. Given the use of thenitrogen-doped layer 306, the layer 312 will include a surface having anitrogen concentration of at least 3×10²⁰ atoms/cc. Once the layer 312is formed, it can be sent for further processing (e.g., masking,etching, etc.). It will be apparent to skilled persons that the p-Silayer 312 formed in accordance with this embodiment has a thicknessequal to that of the semiconductor layer 300 and the nitrogen-dopedlayer 306 combined. As a non-limiting example, if a 50-nm-thick p-Silayer is desired, the method may be carried out by forming and annealinga 10-nm-thick a-Si nitrogen-doped layer 306 on a 40-nm-thick a-Sisemiconductor layer 300.

Another embodiment of the method of FIG. 1 will now be described withreference to FIGS. 3A to 3C.

As with FIG. 2A, FIG. 3A shows a cross-section segment of an examplesemiconductor layer 400, which has a product region 402 and which isdisposed on a substrate 404. In this form, step 200 of FIG. 1 is carriedout by ion implanting or plasma doping nitrogen into the upper portion406 of the semiconductor layer 400. This results in the upper portion406 of the semiconductor layer 400 being converted into a nitrogen-dopedlayer 406. In other words, in this embodiment, the nitrogen-doped layer406 forms part of the semiconductor layer 400 but it can bedistinguished from the semiconductor layer 400 by virtue of the nitrogenconcentration in the nitrogen-doped layer 406. The nitrogenconcentration and depth (thickness) of the nitrogen-doped layer 406 maybe controlled by controlling the energy used in the ion implantation orplasma doping process. As before, the nitrogen concentration is at least3×10²⁰ atoms/cc.

Steps 202 and 204 of FIG. 1 are then carried out on the nitrogen-dopedlayer 406. Specifically, a first area of the nitrogen-doped layer 406 issubjected to laser annealing 408, and a second area of thenitrogen-doped layer 406 is subjected to laser annealing 410. As before,the first and second areas overlap in the product region 402, and thelaser annealing process results in a crystallized layer 412. Where a-Siis used to form the semiconductor layer 400, the result of annealing isa p-Si layer 412. It will be apparent to skilled persons that the p-Silayer 412 in this embodiment will have a thickness substantially equalto that of the semiconductor layer 400. As a non-limiting example, if a50-nm-thick p-Si layer is desired, the method in this embodiment may becarried out using a 50-nm-thick semiconductor layer 400.

In another example of the method of FIG. 1, step 200 further comprisesforming a nitrogen-doped layer that includes oxygen, the oxygenconcentration being in the range of 3×10²¹ to 7×10²² atoms/cc. Moreparticularly, the oxygen concentration may be in the range of 5×10²¹ to5×10²² atoms/cc. The oxygen concentration may be provided by forming thenitrogen-doped layer using nitrous oxide (N₂O) gas in a CVD process, orotherwise in the presence of oxygen (e.g., oxidation by air).Alternatively, the oxygen concentration may be provided by ionimplanting or plasma doping oxygen into the nitrogen-doped layer.

It should be noted that the oxygen concentration may be obtained before,after, or at the same time as doping the layer with nitrogen.

As outlined earlier, the use of the nitrogen-doped layer in the presentmethod allows a reduction in certain defects that are typically observedat the overlap region after the laser annealing process. In particular,the overlap region of a conventional laser annealing method typicallyexhibits excessive grain protrusion (i.e., surface roughness), whichadversely affects the electrical and physical characteristics of thatregion of the annealed semiconductor layer. Specifically, whereexcessive grain protrusion is generated, it becomes difficult to coverthe annealed semiconductor layer with a thin insulating layer to form asemiconductor device. This, in turn, may result in an electrical shortbetween the annealed semiconductor layer and a conductive layer (e.g., agate electrode) via the insulating layer, which is disposed between theannealed semiconductor layer and the conductive layer. This arrangementof layers of a semiconductor device will be described in further detaillater with reference to FIG. 7. Given the above drawbacks, conventionallaser annealing is restricted in that the overlap region must beprovided outside of product regions.

As in a comparative example shown in FIG. 4A, for example, in the casewhere an array 500 having 3×3 product regions 502 is laser-annealed andthere is the restrictive condition that all overlap regions 506 are tobe provided outside the product regions 502, the length of the laserbeam 504 is restricted to x₁ and the beam 504 will have to be scannedthree times over three areas 508, 510 and 512 to cover the entire array.

Where a nitrogen-doped layer is used in accordance with the techniquesdescribed above, as shown in FIG. 4B, the annealing may be carried outin a low oxygen atmosphere, thus allowing the grain protrusion of theoverlap region 506 to be reduced to an acceptable level. This, in turn,makes it feasible for the overlap region 506 to fall within one or moreproduct regions 502 and for the annealing process to be completed bysetting the laser beam to a length x₂ greater than x₁ and by scanningthe beam in fewer scans (two scans over first area 514 and second area516 in the example of FIG. 4B, as opposed to three scans over areas 508,510 and 512 in the example of FIG. 4A). In this embodiment, the methodallows annealing to be carried out in less time and using fewer laserscans. Therefore, the production time is reduced, leading to animprovement in production efficiency and a reduction in productioncosts.

An example of the reduction in defects in the form of grain protrusionsin the overlap region will now be described with reference to FIGS. 5A,5B, 6A and 6B. FIG. 5A shows a diagrammatic side view of an excimerlaser beam 600 with the energy density of 300 mJ/cm² scanned over afirst area (consisting of regions 1, 2) of a semiconductor layer 604,and a laser beam 602 with the energy density of 300 mJ/cm² scanned overa second area (consisting of regions 2, 3) of the semiconductor layer604, according to the comparative example. In the comparative exampleillustrated, the semiconductor layer 604 is a 50-nm-thick a-Si layerthat has been deposited on a glass substrate by introducing silane gas(together with argon (Ar) gas as a carrier) in a CVD chamber containingthe glass substrate.

FIG. 5B shows the results from conventionally laser-annealing thesemiconductor layer 604, with the x-axis representing the regions shownin FIG. 5A and the y-axis representing grain protrusion height innanometers. The y-axis includes an upper limit marker (e.g., 20 nm),which represents the maximum grain protrusion height that can beadequately covered by the thin insulating layer (708 in FIG. 7) thatwill be subsequently formed on the p-Si layer (702 in FIG. 7). Asoutlined earlier, the presence of grain protrusions that exceed theupper limit typically results in the overlap region exhibitingundesirable electrical and physical characteristics.

From FIG. 5B, it is clear that the overlap region (region 2) in aconventional annealing process exhibits grain protrusion over the upperlimit.

Referring now to FIG. 6A, the semiconductor layer 604 has anitrogen-doped layer 608, in accordance with the present method. In theembodiment illustrated, the semiconductor layer 604 and nitrogen-dopedlayer 608 are sequentially formed as follows:

1. A glass substrate is supported on a susceptor in a CVD chamber.

2. The air in the CVD chamber is exhausted, and silane gas (togetherwith Ar gas as carrier) is introduced in the CVD chamber containing theglass substrate.

3. A 40-nm-thick a-Si layer is deposited on the glass substrate to formthe semiconductor layer 604.

4. N₂O gas is introduced in the CVD chamber together with silane gas toform a 10-nm-thick doped a-Si layer 608 having nitrogen atoms and oxygenatoms at a concentration of 2×10²¹ and 2×10²² atoms/cc, respectively.

Thereafter, in the same manner as described above, the first region andsecond region of the doped a-Si layer 608 are laser-annealed by thelaser beams 600 and 602, respectively.

As shown in FIG. 6B, the result of using the nitrogen-doped layer 608 isthat the grain protrusion across the overlap region (region 2) isreduced to below the upper limit. For instance, the grain protrusionheight in the overlap region is 15 nm, which is lower than the 20-nmupper limit. The value of the grain protrusion height of the annealedsemiconductor layer as a whole is therefore less than the upper limit of20 nm. Furthermore, the grain size of the overlap region is larger than0.2 μm (e.g., around 0.35 μm), and is substantially uniform. With suchacceptable grain size and protrusions, the overlap region no longerexhibits undesirable electrical and physical properties, thus allowingthe overlap region to be formed in one or more product regions withoutadversely affecting the electrical and physical characteristics of theproduct regions. It should be noted that the use of the nitrogen-dopedlayer 608 results in the p-Si layer (i.e., after laser annealing) havinga surface with a nitrogen concentration of 1×10²¹ atoms/cc, and anoxygen concentration of 1×10²² atoms/cc.

Next, a description is given of an array substrate of a liquid crystaldisplay device as an example of the semiconductor device formed from thesemiconductor layer annealed using the present method, and a fabricationmethod thereof. FIG. 7 shows an array substrate 700 for an LCD includinga co-planar type TFT as the semiconductor device.

The array substrate 700 includes a transparent insulating substrate 721such as a glass substrate, and an undercoat layer 722 which is formed onthe insulating substrate 721 and functions to prevent impurity diffusionfrom the insulating substrate 721. A semiconductor layer 724 of p-Si,which is patterned in a predetermined shape, is formed on the undercoatlayer 722. The crystalline structure of the semiconductor layer 724forms a TFT active layer 702 including a source region 702 a, a drainregion 702 b, and a channel region 702 c sandwiched between the sourceand drain regions. A gate insulation film 726, which is made of, e.g.,SiO₂ or TEOS, is formed on the TFT active layer 702 and undercoat layer722. The TFT active layer 702 has a first surface and a second surface.When the TFT active layer 702 is deposited on the glass substrate, thesecond surface is located on the insulating substrate 721 side, and thegate insulation film 726 is provided on the first surface of the TFTactive layer 702.

The TFT active layer 702 (and more specifically, the first surface ofthe layer 702) has a nitrogen concentration of 1×10²¹ atoms/cc. In moregeneral terms, the first surface of the TFT active layer 702 may have anitrogen concentration in the range of 3×10²⁰ to 1×10²² atoms/cc, morespecifically 5×10²⁰ to 5×10²¹ atoms/cc. As described above, the nitrogenconcentration is the result of initial doping of the nitrogen-dopedlayer.

In addition, the TFT active layer 702 (and more specifically the firstsurface of the layer 702) may have an oxygen concentration of 1×10²²atoms/cc. In more general terms, the first surface of the TFT activelayer 702 may have an oxygen concentration in the range of 3×10²¹ to7×10²² atoms/cc, more specifically 5×10²¹ to 5×10²² atoms/cc. In oneform, the first surface of the TFT active layer 702 has a nitrogenconcentration of at least 3×10²⁰ atoms/cc.

A gate electrode 710 of a metal, such as aluminum (Al), an aluminum (Al)alloy or a MoW alloy, is formed on the gate insulation film 726. Thegate electrode 710 is opposed to the channel region 702 c of the TFTactive layer 702, with the gate insulation film 708 being interposed. Aninterlayer insulation film 728 of SiNx is formed to cover the gateinsulation film 726 and gate electrode 710. Contact holes 90 and 91 areformed in the interlayer insulation film 728 and gate insulation film726. A source electrode 704 and a drain electrode 706, which are made ofa metal such as aluminum or an aluminum alloy, are formed in the contactholes 90 and 91. The source electrode 704 and drain electrode 706 areelectrically connected to the source region 702 a and drain 702 b of theTFT active layer 702, respectively. Of these parts, a thin-filmtransistor (TFT) 701 is composed.

A protection layer 730 is formed on the interlayer insulation film 728,and a pixel electrode 711, which is made of, e.g., a transparentelectrically conductive film, is formed on the protection layer 730. Thepixel electrode 711 is electrically connected to the drain electrode 706of the TFT 701 via a contact hole that is formed in the protection layer730. Besides, the array substrate 700 includes a signal line, a scanningline, etc., which are not shown.

In one form, the TFT 701 controls the pixel electrode 711 that isdisposed on the TFT 701 via an insulating layer in an LCD, andconstitutes one of a plurality of TFTs formed in each product region,where each product region makes up an LCD display region or an LCDpanel. In another form, the TFT 701 controls pixels in an OLED displaydevice, and constitutes one of a plurality of TFTs formed in eachproduct region, where each product region makes up an OLED displayregion or an OLED panel.

Next, a description is given of a method of fabricating an arraysubstrate including a TFT with the above-described structure.

As shown in FIG. 8A, an undercoat film 722 is formed on a transparentsubstrate 721 such as a glass substrate. As the undercoat film 722, useis made of an SiO₂ film which is formed by a CVD method or a sputteringmethod. Alternatively, as the undercoat film 722, use may be made of athin film of SiNx or a double-layer thin film of SiNx and SiO₂.

Subsequently, a polysilicon (p-Si) film serving as a semiconductor layer724 is formed on the undercoat film 722. In the case of forming thisp-Si film, for example, an a-Si film is formed by a film formationmethod such as a plasma CVD method or a sputtering method, followingwhich nitrogen doped a-Si film is formed on the a-Si film, or nitrogenis ion-implanted or plasma-doped in an upper portion of the a-Si film.By this fabrication step, the upper portion of the a-Si film is formedof a nitrogen-doped layer. Specifically, in this step, although thenitrogen-doped a-Si layer forms part of the semiconductor layer 724, itcan be distinguished from the semiconductor layer by the nitrogenconcentration in the nitrogen-doped a-Si layer. The nitrogenconcentration and depth (thickness) of the nitrogen-doped a-Si layer iscontrolled by adjusting the thickness at the forming the nitrogen-dopeda-Si layer, the flow rate of the introducing gas, or the energy in theion implantation or plasma doping process. The nitrogen concentrationis, e.g., 3×10²⁰ atoms/cc.

In the following step, as shown in FIG. 8B, the semiconductor layer 724,in which the nitrogen-doped a-Si layer is formed, is laser-annealed toform a p-Si film. At this time, laser annealing 408 is performed on afirst region of the semiconductor layer 724 where the nitrogen-dopeda-Si layer is formed, and further laser annealing 410 is performed on asecond region of the semiconductor layer 724. The first region andsecond region overlap in the product region, and a p-Si film is formedby the laser annealing step.

Next, as shown in FIG. 8C, the p-Si film is patterned to form aplurality of island-shaped semiconductor layers 724. Further, a gateinsulation film 726, which is made of an SiO₂ film, is formed on theundercoat film 722 and the semiconductor layer 724 by, e.g., plasma CVD.As the method of forming the gate insulation film 726, the plasma CVDmethod may be replaced with some other CVD method, such as anatmospheric pressure CVD method, an LPCVD method, an ECR plasma CVDmethod or a remote plasma CVD, or a sputtering method. As the materialgas, use may be made of TEOS.O₂ gas or SiH₄.O₂ gas.

Following the formation of the gate insulation film 726, the gateinsulation film may be annealed, for example, under such conditions thatthe annealing is performed in a nitrogen atmosphere at 600° C. for fivehours.

Subsequently, as shown in FIG. 8D, for instance, a low-resistance metalfilm of a molybdenum-tungsten alloy (MoW) or aluminum (Al) or animpurity-doped polycrystalline silicon film is formed on the gateinsulation film 726, and this film is patterned in a predeterminedshape, thereby forming a gate electrode 710.

After the gate electrode 710 having a predetermined shape is formed,phosphorus (P), which is n-type impurity, is ion-implanted in thesemiconductor layer 724, as shown in FIG. 9A, in a self-alignmentfashion by using the gate electrode 710 as a mask. Thereby, a sourceregion 702 a and a drain region 702 b are formed in the p-Si film. Then,the ion-implanted phosphorus is activated by annealing such as laserannealing or thermal annealing. Thus, a TFT active layer 702 includingthe source region 702 a, drain region 702 b, and channel region 702 csandwiched therebetween is formed.

In the case of fabricating a P-channel TFT, P-type impurities, such asboron, are ion-implanted in the semiconductor layer 724.

Then, as shown in FIG. 9B, an interlayer insulation film 728 withinsulating properties is formed on the entire surfaces of the gateinsulation film 726 and gate electrode 710. Contact holes 712 a and 712b, which communicate with the source region 702 a and drain region 702 bof the TFT active layer 702, are formed in the interlayer insulationfilm 728.

As shown in FIG. 9C, a metal film of, e.g., Al, is formed over theentire surface of the interlayer insulation film 728 so as to fill thecontact holes 712 a and 712 b. Then, this metal film is patterned toform a source electrode 704 and a drain electrode 706. Thereby, the TFT701 is obtained.

Following the above, in order to protect the TFT 701 from, e.g.,adsorption of moisture, a protection film of, e.g., a silicon nitridefilm is formed. Further, a contact hole 714, which communicates with thedrain electrode 706, is formed in the protection film 730. A transparentelectrically conductive film of, e.g., ITO is formed over the entiresurface of the protection film 730 so as to fill the contact hole 714,and this transparent electrically conductive film is patterned to form apixel electrode 711. Thereby, an array substrate 700 including aplurality of TFTs 701 is obtained.

The present invention is not limited directly to the embodimentdescribed above, and its components may be embodied and modified withoutdeparting from the spirit of the invention. Further, various inventionsmay be made by suitably combining a plurality of components described inconnection with the foregoing embodiment. For example, some of thecomponents according to the foregoing embodiment may be omitted.Furthermore, components according to different embodiments may becombined as required.

For example, in the above-described laser annealing method, the stepsare carried out in a predetermined order. This method, however, may bemodified such that the steps are carried out in an order different fromthe order as described above. Besides, this method may be modified so asto perform additional steps. The feature that the overlap region ispresent in one or more product regions is not limited to the embodimentin which the overlap region is formed in the plural product regions. Itshould suffice if at least a part of the overlap region is formed in theproduct region.

Furthermore, the nitrogen-doped layer has been described as beingeffective in reducing grain protrusions, but the nitrogen-doped layer isnot limited to this use. For example, the nitrogen-doped layer may beused in order to reduce defects such as ablation. The above-describedmodifications fall within the scope of the present invention.

1. A method of laser annealing a non-single-crystalline semiconductorlayer, the non-single-crystalline semiconductor layer including aproduct region, the method comprising: forming a nitrogen-doped layer onthe non-single-crystalline semiconductor layer, the nitrogen-doped layerhaving a nitrogen concentration of at least 3×10²⁰ atoms/cc; irradiatinga first area of the nitrogen-doped layer in a low oxygen environmentwith a laser beam; and irradiating a second area of the nitrogen-dopedlayer in a low oxygen environment with a laser beam, a part of thesecond area overlapping with the first area, thereby forming alaser-annealed semiconductor layer of which a root mean square (rms)value of grain protrusion height is less than 20 nm.
 2. The method ofclaim 1, wherein the nitrogen-doped layer is formed by doping nitrogeninto the non-single-crystalline semiconductor layer.
 3. The method ofclaim 1, wherein the non-single-crystalline semiconductor layer and thenitrogen-doped layer are sequentially deposited by using a chemicalvapor deposition (CVD) process.
 4. The method of claim 3, wherein theCVD process is carried out by introducing SiH₄ gas at a first flow rateand N₂O gas at a second flow rate into a CVD chamber.
 5. The method ofclaim 4, further comprising controlling a concentration of nitrogen inthe nitrogen-doped layer by controlling one or both of the first flowrate and the second flow rate.
 6. The method of claim 2, wherein thestep of doping nitrogen into the non-single-crystalline semiconductorlayer is carried out using an ion implantation or plasma doping process.7. The method of claim 1, wherein the nitrogen-doped layer has athickness in the range of 1 to 30 nm.
 8. The method of claim 7, whereinthe nitrogen-doped layer has a thickness in the range of 5 to 15 nm. 9.The method of claim 1, wherein the nitrogen-doped layer has a nitrogenconcentration in the range of 3×10²⁰ to 1×10²² atoms/cc.
 10. The methodof claim 9, wherein the nitrogen concentration is in the range of 5×10²⁰to 5×10²¹ atoms/cc.
 11. The method of claim 1, wherein thenitrogen-doped layer is also doped with oxygen and has an oxygenconcentration in the range of 3×10²¹ to 7×10²² atoms/cc.
 12. The methodof claim 11, wherein the oxygen concentration is in the range of 5×10²¹to 5×10²² atoms/cc.
 13. The method of claim 1, wherein thenon-single-crystalline semiconductor layer includes a plurality ofproduct regions, each comprising a display region of a display device.14. A method of laser annealing a non-single-crystalline semiconductorlayer, the non-single-crystalline semiconductor layer including aproduct region, the method comprising: forming a nitrogen-doped layer onthe non-single-crystalline semiconductor layer having a nitrogenconcentration of at least 3×10²⁰ atoms/cc and an oxygen concentration inthe range of 3×10²¹ to 7×10²² atoms/cc; irradiating a first area of thenitrogen-doped layer in a low oxygen environment with a laser beam;irradiating a second area of the nitrogen-doped layer in a low oxygenenvironment with a laser beam, a part of the second area overlappingwith the first area, thereby forming a laser-annealed semiconductorlayer of which a root mean square (rms) value of grain protrusion heightis less than 20 nm.
 15. The method of claim 14, wherein thenitrogen-doped layer is formed by doping nitrogen and oxygen into thenon-single-crystalline semiconductor layer.
 16. The method of claim 14,wherein the non-single-crystalline semiconductor layer and thenitrogen-doped layer are sequentially deposited by using a chemicalvapor deposition (CVD) process.